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Genetics
3.3 Meiosis
10.1 Meiosis (AHL)
Recall Mitosis
• Purpose:
• Process:
• Results:
Sexual Reproduction
•
•
•
•
What is sexual reproduction?
Why is it beneficial?
What kinds of cells are involved?
How are these cells formed?
Diploid vs Haploid
• Diploid nuclei have pairs of homologous
chromosomes.
• Haploid nuclei have one chromosome of each
pair.
Meiosis
• Meiosis is the process by which sex cells, or
gametes, are formed.
▫ In humans this takes place in the testes and
ovaries
• Meiosis involves 2 stages of cell division.
• In the end, daughter cells contain half the
chromosome number of the parent cell =
haploid.
• Halving of the chromosome number allows a
sexual life cycle with fusion of the gametes.
Stages of Meiosis:
(Make a table as shown on the board)
• Meiosis I:
▫ Purpose: Reduce chromosome number
▫ Process: Prophase I, Metaphase I, Anaphase I, Telophase I
▫ Results: Two Daughter cells each containing half the number of
chromosomes (still in the form of sister chromatids).
• Meiosis II:
▫ Purpose: Separate sister chromatids
▫ Process: Prophase II, Metaphase II, Anaphase II, Telophase II
▫ Results: Total of Four haploid Daughter cells (which may
become gametes).
Crash Course Biology Video:
https://www.youtube.com/watch?v=qCLmR9-YY7o
Meiosis I
• Prophase I:
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▫
▫
▫
Nuclear membrane dissolves
Centrioles split and move to opposite poles
Spindle fibers form
Chromosomes come together in homologous pairs
 Each chromosome is composed of a pair of sister
chromatids
 The whole structure is referred to as a tetrad because
it contains 4 chromatids
▫ The intertwined chromatids may break and
exchange segments of genetic material =
crossing over
Meiosis I
• Metaphase I:
▫ Homologous chromosomes attach to spindle
fibers
▫ In pairs, they line up along the equatorial plate
Meiosis I
• Anaphase I:
▫ “Reduction division” occurs
▫ One member of each Homologous pair moves
towards either pole.
 Each chromosome still contains sister chromatids
Meiosis I
• Telophase I:
▫ Membranes form to create 2 new nuclei
▫ Each of the daughter nuclei contains one member
of each chromosome pair.
▫ Telophase is followed by cytokinesis, the division
of the cytoplasm.
Meiosis II
• Prophase II:
▫ Nuclear membrane dissolves
▫ Spindle fibers form
Meiosis II
• Metaphase II:
▫ Chromosomes, each with two chromatids, line up
along the equatorial plate.
Meiosis II
• Anaphase II:
▫ Attachment between the sister chromatids is
broken
▫ Individual chromatids move to opposite poles
Meiosis II
• Telophase II:
▫ Nuclear membranes form
▫ Form haploid nuclei have been produced
▫ Telophase is followed by cytokinesis.
Meiosis
• Complete the worksheets
Meiosis
• Complete the practical activity
▫ P 110 and 111 in Bio 11 textbook
Bivalent formation and crossing over
• The early stages of meiosis
involve pairing of
homologous chromosomes
and crossing over followed
by condensation.
• Bivalent = a pair of
homologous chromosomes
• Synapsis = pairing process
• Junction = the area where
chromosomes break and
rejoin
• Chiasmata = the point of
connection
Random Orientation of the bivalents
• Orientation of pairs of homologous
chromosomes prior to separation is
random.
• Orientation = the pole to which
each chromosome is attached
depends on the way it is facing.
• For every additional bivalent, the
number of possible chromosome
combinations in a cell produced by
meiosis doubles (2n).
▫ Calculate the number of possible
combinations in humans
Non-disjunction
• Non-disjunction = when
homologous chromosomes fail to
separate in anaphase.
• Both of the chromosomes move to
one of the poles.
• The result is a gamete with either
an extra chromosome or deficient
in a chromosome.
• If this gamete goes on to be
fertilized, the result will be an
individual with 45 or 47
chromosomes.
• Example: Down Syndrome